In comparison with a fluorescent tube (including a hot cathode-ray tube and a cold cathode-ray tube), a LED light source is characterized by its reduced environmental burden because it is free from mercury, and is characterized by its excellent color reproducibility, excellent responsibility, wider controllable range of luminance, and longer life. Because of these characteristics, liquid crystal display devices, which are mainly small in size, employ such LED light sources for a backlight and are coming into wide use. In recent years, large-sized liquid crystal display devices also employ such LED light sources as disclosed, for example, in Japanese Unexamined Patent Application Publication No. 2007-532949 (as Japanese translation of PCT International Publication No. WO2005/101807), Japanese Unexamined Patent Application Publications (JP-A) Nos. 2007-123153 and 2004-29141, and PCT International Publication No. WO2009/110129.
Light emitting diodes (LEDs), so-called “cold light sources”, belong among semiconductor devices and use a light-emitting phenomenon based on a combination of an electrode and a hole. Therefore, most of LEDs are composed of semiconductor having a band gap corresponding to wavelengths of visible light (for example, inorganic materials including GaN as the base material). Further, organic light-emitting diodes called OLEDs are now being researched and developed, and a part of them has already been put to practical use. There are LEDs with various colors, which differ from conventional fluorescent tubes. For example, not only white LEDs (White Light Emitting Diodes or W-LEDs) but also LEDs adjusted to emit light in any one of various colors, such as red, green, blue and yellow, have been on the market so as to meet various kinds of demands.
LED light sources have small aging-deterioration coming from the use environment and can emit bright light over a long period of time, but have a drawback that their optical properties (that is, the chromaticity of emitted light) fluctuate greatly over a long period of time. Further, single-color LEDs, such as red (R), green (G) and blue (B) LEDs, generally have less light-emission efficiency than that of white LEDs (W-LEDs). To obtain white light of the same luminance, a use of a white LED can reduce power consumption in comparison with a use of a mixture of light in red (R), green (G) and blue (B) emitted by single-color LEDs. Therefore, most liquid crystal display device uses W-LEDs.
However, LED light sources, especially white LEDs, significantly change in color after a long time use, which is a great problem for liquid crystal display devices using LED light sources in a backlight. On the other hand, with increase of the life of various products, liquid crystal display devices are also expected to maintain their properties for a long time in the market (especially in the industrial field). In other words, liquid crystal display devices are expected to have a conventional structure in which a liquid crystal display panel with a simple RGB-pixel structure and further to maintain both of decreased power consumption and high color-reproducibility for a long time, not only at the beginning of use.
In view of such a background, there have been proposed various technologies about liquid crystal display devices. For example, JP-A No. 2007-532949 discloses a technology to realize reduced power consumption of a liquid crystal display device, that is, a structure and a method to use RGBW color filters (also referred to as CFs) in a liquid crystal display panel to have a greater light transmittance in comparison with a liquid crystal display using RGB color filters. However, in the disclosed structure, the liquid crystal display panel transmits white light (W) additionally to light in R, G and B, which causes a problem that the color reproducibility greatly deteriorates. Further, the disclosed technology needs not only a great change of the structure of the liquid crystal display panel (especially, in the pixel structure) but also additional components such as a drive circuit and a conversion circuit for white light, which makes the system complicated and increases the number of components. Accordingly, this structure further makes a problem about an increase of the cost.
JP-A No. 2007-123153 discloses the following liquid crystal display device. The liquid crystal display device achieves high color-reproducibility without modifying a liquid crystal display panel, by using general RGB color filters, three color (red, green and blue) LED light sources, and a feedback section including a color sensor. JP-A No. 2004-29141 also discloses a liquid crystal display device using three color (red, green and blue) light emitting diodes. The liquid crystal display device automatically adjusts the white balance and luminance by using a current-adjustment control circuit and a data-correction control circuit which corrects data of the light quantity obtained from photo-detectors that detects red light, green light and blue light.
By employing those technologies, liquid crystal display devices can maintain the high color-reproducibility for a long time. However, LEDs each independently emitting red light, green light and blue light form white light with less light-emission efficiency in comparison with a case that white light is formed with only white LEDs (W-LEDs), and it causes a problem about an increase of the power consumption. Further, if the LEDs deteriorate gradually during a long time use, such situation needs an increase of the quantity of electric current for the LEDs in order to maintain the constant luminance. It results in an increased power consumption, which is a problem.
WO 2009/110129 discloses a liquid crystal display device employing a RGB liquid crystal display panel and a backlight using four color light sources (for example, RGBW light sources). The disclosed technology of the document can provide a high color-reproducibility by conducting a proper control separately on the four color light sources in the backlight, but does not conduct feedback processing using color sensors. The disclosed technology uses backlight data for four colors to be target values. The backlight data is used for calculating luminance values of light in RGB colors for the light source with each color, on the basis of tristimulus values calculated by subtracting the tristimulus values (arbitrary luminance values relative to the maximum gradation level) for the fourth color from target tristimulus values. WO 2009/110129 does not disclose any countermeasure for fluctuation of the light intensity of the four color LEDs in the backlight and deterioration of the LEDs both of which can be caused after a long time use. In other words, it is obvious that the color reproducibility of such a structure can greatly deteriorate after a long time use.
Further, since general input image signals are composed of signals of three (RGB) color-components, the technology disclosed in WO 2009/110129, which uses light sources consisting of four color LEDs in the backlight, needs a calculation circuit for converting the RGB signals into four-color-based signals. Such a calculation circuit consumes much power and harms to achieve the reducer power consumption.
The present invention seeks to solve the problems.